The Stroop Effect – How it Works and Why

To see and interact with the world, we first need to understand it. Visual processing is one way we do this, and is composed of many parts. When we see an object, we don’t just see its physical attributes, we also comprehend the meaning behind them. We know that a chair needs legs because the seat needs to be raised, we know that the wood comes from trees, we know we could sit in it, and so on. There is information that we process about the things we see without even being aware of that processing.

So when John Ridley Stroop asked people to read words on a sheet of paper in 1929, he knew that their automatic processing would come into play, and could offer a breakthrough insight into brain function. Research from as early as 1894 had shown that associations of even nonsense syllables would become embedded into a person’s understanding, and could interfere with how they processed and recalled these syllables, despite no real meaning being attached to them. It was therefore clear, even in the beginnings of contemporary psychological research, that associations are powerful and pervasive.

What is the Stroop Effect?

Stroop’s innovation was to show, clearly and definitively, that our embedded knowledge about our environment impacts how we interact with it. His research method is now one of the most famous and well-known examples of a psychological test, and is elegant in its simplicity.

First, the participant reads a list of words for colors, but the words are printed in a color different to the word itself. For example, the word “orange” would be listed as text, but printed in green. The participant’s reading time of the words on the list is then recorded. Next, the participant has to repeat the test with a new list of words, but should name the colors that the words are printed in. So, when the word “orange” is printed in green, the participant should say “green” and move on to the next word.

First, time yourself while you read the following text, ignoring the colors the words are printed in.

BLUE ORANGE YELLOWREDPURPLE

PINK BLUEBLACKPURPLE GREEN

ORANGEBLACKYELLOWPINK RED

BLUEPINK ORANGE BLACKBLUE

Now time yourself while you state the colors of the following words, ignoring the actual text (as best as you can!).

YELLOWREDPINKBLUEGREEN

PURPLEYELLOW BLUE BLACK PINK

BLUE REDGREENORANGEPINK

BLACK RED YELLOW PURPLEBLUE

In most cases, it takes longer to state the colors of the words, rather than to read the text they are printed in, despite the incongruence being essentially the same across both lists (i.e. both show words in the wrong color). It appears we are more influenced by the physical text than than the text color.

Why does this happen?

What this reveals is that the brain can’t help but read. As habitual readers, we encounter and comprehend words on such a persistent basis that the reading occurs almost effortlessly, whereas declaration of a color requires more cognitive effort. When there is a conflict between these two sources of information, our cognitive load is increased, and our brains have to work harder to resolve the required difference. Performing these tasks (preventing reading, processing word color, and resolving information conflict) ultimately slows down our responses, and makes the task take longer.

There are a few theories that slightly differ in their definitions of the Stroop Effect, yet their differences mostly lie in which part that they emphasize. For example, one theory emphasizes that the automaticity of reading as the principal cause of Stroop interference, while another emphasizes the mental prioritizing which we perform when reading, as compared to defining colors. While differences in theories may therefore exist, all essentially converge on the central premise that reading is a simpler and more automatic task than stating colors, and that a conflict between the two will increase the time needed for processing.

What can we use it for?

Using this paradigm, we can assess an individual’s cognitive processing speed, their attentional capacity, and their level of cognitive control (otherwise known as their executive function). These skills and facets are implicit in so many ways in which we interact with the world, suggesting that this test reveals a brief – yet incisive – view into human thought and behavior.

The test is also used in a variety of different ways to the original, in an effort to exploit the experimental setup to reveal more about a clinical population, for example. Even neurodevelopmental disorders such as schizophrenia and autism have been examined with the Stroop test.

Furthermore, there are several variations and differing implementations of the test available, allowing different aspects of cognition to be honed in on. One of these variations is the “emotional Stroop test” in which participants complete both the original Stroop, and a version which has both neutral and emotionally charged words. The resulting text features words such as “pain” or “joy” amongst everyday words. Research has shown that anxious people were likely to experience more interference (i.e. more time spent declaring word color) with emotionally charged words, suggesting a preponderance of the emotional word content.

Experimental designs like this allow researchers to target and observe cognitive processes that underlie explicit thought. The test reveals the working of non-conscious brain function and reduces some of the biases that can otherwise emerge in testing.

Other experimental setups utilize the lessons of the Stroop Effect – that incongruent information will require more mental resources to resolve correctly – with numbers, rather than words. Termed the “Numerical Stroop Effect”, this experiment has shown that presenting numbers of incongruent sizes next to each other will slow down reading and comprehension. For an example, see the image below:

Examples of the different test types that are used in the Numerical Stroop.

This experiment shows that, with all else being controlled for, incongruence in numerical size will cause the greatest interference, increasing the delay in comprehension. An interesting feature with the Numerical Stroop is that the interference is found for both types of incongruence – when the numbers are incongruent with size, then a delay is shown for reporting the size, as well as for reporting the numbers. This effect reveals that the automatic processing is not just limited to words, suggesting that the brain looks for normal patterns in a variety of presented stimuli, as it appears to struggle when this doesn’t occur.

How can the Stroop test be used?

The Stroop test can be simply administered with a basic experimental setup. At its most fundamental, all you need is an image of the Stroop test words, a stopwatch, and someone to record the time and answers (and a willing participant!). However, if you want to gain more insights from the data, there are plenty of ways to take the test further. With iMotions you can simply set up and present the Stroop test, while also expanding the data collection possibilities. Using the survey function, the test can be quickly and simply added. This can be done with either the built-in iMotions survey tool, or with the Qualtrics survey tool, which allows even more metrics to be taken into account.

The ability to record from various synchronized biometric devices opens up new avenues for research. For example, with an eye-tracking tool, you can examine exactly how long each participant looks at each word, and their precise speed of comprehension. Using areas of interest (AOIs) can be of particular use as this allows you to analyze specific parts of the scene in isolation, or compared to the data for the scene as a whole, or even with other AOIs. It’s then possible to determine which words demanded the most visual attention, allowing you to accurately dissect the data in fine detail.

Below are a few examples of that idea in practice, each of which took only minutes to set up and start.

First, we’ve added an image of a Stroop test to the survey function – one version is essentially the same as the original, while another has neutral words mixed with food related words. This version of the Stroop test would require that the participant verbally declare the color of each word – audio recording could help in accurately measuring participant responses. We have also included an example using a multiple choice paradigm that is detailed below, and using the Qualtrics survey function below that.

The normal Stroop test inserted as a survey image into iMotions.

A modified Stroop test inserted as a survey image into iMotions.

After we’ve set up eye-tracking and added a participant list, we can add AOIs to the words, so that we can view and analyze data for each. Below is an image of how this looks:

The Stroop test in iMotions with AOIs placed over the color words.

After running through a few participants, we can start to visualize and analyze their data, producing both detailed AOI data, and heatmaps showing overview data. Below are examples of what this data could look like. Of course, more detailed data is available to export and analyze, if desired.

Data displayed in iMotions showing the time-to-first-fixation (TTFF), the time spent in seconds looking at the AOI (which is only shown to one decimal point in the image above), and the ratio of participants who viewed the AOI.

A heatmap showing the level of fixation across the words shown in the Stroop test.

Alternatively, we can insert each word of the Stroop test within the survey setup, and use the keyboard input function for the participant to answer each word color. This would also allow us to investigate the error rate in a more systematic manner. This is shown across the two images below.

The survey setup with an incongruent word-color stimuli. Several of these surveys can be quickly arranged for multiple tests.

How the above survey appears to the participant. The participant is required to choose one of the predefined colors before advancing to the next question.

Within this paradigm, eye movements can also be measured, providing information about the amount of time taken to process the information. The approach may take longer for each participant, and remembering the keyboard-color combinations may encumber their cognitive processing (although this shouldn’t present a problem if this approach is used with the correct controls), however it does allow a finer dissection of eye movement for each word, and also informs us about the error rate from incorrect answers.

Using Qualtrics

Finally, we can see how this test is implemented in iMotions using the Qualtrics survey function. This is easily implemented, and appears in a similar way to the above surveys that are built by iMotions. One of the advantages of using Qualtrics is that feedback to participant answers can be immediately provided, should this be desired. The following image shows how the stimulus presentation appears on screen.

Qualtrics implementation of the Stroop test.

The participant can then click on the corresponding color to answer the question. If an incorrect answer is chosen, the response would be shown as below.

Feedback for participant in Qualtrics.

The participant can then proceed to complete other questions, and their answers will be recorded, allowing later analysis and visualization of the results.

With all of the information completed and data analyzed, we can now start to discern which words showed the greatest amount of Stroop interference (the latency produced when naming the color that the word is printed in). Having several paradigms with different colors, words, and with only blocks of colors will provide more baseline information and control for experimental error. Ultimately this gives a good basis for the participant data to be normalized, and compared with more validity. We can now test if there is any difference with the words of interest and potentially start to draw conclusions about the implicit thoughts of participants (with the example above, it could be that participants who are hungrier would spend a longer duration in naming the colors of the words, suggesting those words are more salient to them).

Conclusion

The Stroop test is a widely-used, well established methodology that reveals various brain functions, and implicit cognitive workings. The original article has now been cited over 13,000 times and that number will surely continue to rise well into the future. With iMotions, it’s easy to start asking questions with the Stroop Task and to get to the answers quickly. Contact us and hear how we can help with your research needs and questions.

Hi there!
I'm the Science Editor at iMotions. I've previously spent my time as a neuroscientist / psychologist, where I found and developed my love for good science. I have a PhD in neuroscience and developmental biology, alongside a bachelor's degree in psychology, and a master's degree in cognitive and computational neuroscience. I'm a big fan of the brain and mind. I believe in the power of well-captured data to provide answers about who we are, what we think, and why we behave in the way that we do.